Reduction of polycyclic aromatic hydrocarbons in tobacco smoke using palladium salts

ABSTRACT

Methods for treating tobacco which result in the reduction of selected compounds from the tobacco smoke, and corresponding smoking articles containing this treated tobacco, are disclosed. In these methods, a solution containing a palladium salt is applied to the tobacco, preferably by spraying. The palladium salt preferably contains nitrate, chloride, acetate, citrate, gluconate, or salicylate as the anion. After application of the solution, the tobacco is preferably dried in an oven, by forced air, or by natural evaporation. The palladium in the treated tobacco results in the reduction in the level of polycyclic aromatic hydrocarbons, especially benzo(a)pyrene, from the total particulate matter in mainstream and sidestream smoke of the treated tobacco. The addition of mannitol and/or urea to the tobacco, and introduction of ventilation in a cigarette containing the treated tobacco, enhance the ability of palladium to reduce the level of polycyclic aromatic hydrocarbons in the smoke.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with tobacco containing at least one additive which reduces the level of polycyclic aromatic hydrocarbon (PAH) in smoke. More particularly, the invention relates to tobacco treated with a palladium salt to improve the quality of smoke by removing PAHs, especially benzo(a)pyrene (BaP), from the total particulate matter in tobacco smoke.

2. Description of the Related Art

It is desirable to remove certain compounds, including PAHs, from smoke. Catalyst systems have been used in the past to reduce the levels of such compounds from tobacco smoke. For example, WO 02/37990 describes a catalyst system employing particles composed of metallic palladium. A major disadvantage with this catalyst system, however, is that it requires activation with nitrate or nitrite, which therefore must be present in the tobacco along with the metallic particles. Moreover, preparation of these metallic particles first requires that palladium ion be reduced to metallic palladium. Finally, such metallic particles are typically lost from tobacco cut filler during the cigarette manufacturing process.

SUMMARY OF THE INVENTION

The present invention concerns a catalyst system useful for removing PAHs, including BaP, from both mainstream and sidestream tobacco smoke. The present invention overcomes the problems associated with the above-noted catalyst system by eliminating the need for (1) particles, (2) reduction of the transition metal, and (3) a nitrate or nitrite activator.

Specifically, the present invention relates to a method for preparing tobacco comprising adding to tobacco a solution containing a palladium salt. In preferred embodiments, the anion of the palladium salt is nitrate, chloride, or the anion of an organic acid. The anion of the organic acid preferably is acetate, citrate, gluconate, or salicylate. The solvent in the solution preferably is water, acetone, ethanol, a mixture of water and acetone, or a mixture of water and ethanol. If a mixture of water and acetone or a mixture of water and ethanol is used, then the concentration of acetone or ethanol in the mixture preferably is from 5% to 15% (v/v).

Preferably, the concentration of paladium in the prepared tobacco preferably is from 0.01% to 0.2% (w/w), more preferably from 0.02% to 0.07% (w/w). Advantageously, the solution further contains mannitol and/or urea, with mannitol preferably being present in the prepared tobacco at a concentration of from 0.5% to 20% (w/w), more preferably from 8% to 12% (w/w), and urea being present in the prepared tobacco at a concentration of from 0.5% to 10% (w/w), more preferably from 2% to 5% (w/w).

In preferred embodiments, the solution is added to the tobacco by spraying. The weight of the solution added to the dry tobacco (i.e., the tobacco before application of the solution) divided by the weight of the dry tobacco to which the solution is added preferably is from 0.2 to 1.0, more preferably from 0.4 to 0.9, most preferably from 0.6 to 0.9. Advantageously, after application of the solution to the tobacco, the tobacco is dried, preferably by forced air or natural evaporation. If drying is done by forced air, then the forced air preferably has a relative humidity of from 50% to 70%.

Preferably, a finite period of time is allowed to elapse between adding the solution to the tobacco and drying the tobacco. Advantageously, this finite period of time is from 5 minutes to 24 hours, preferably from 15 minutes to 3 hours, more preferably from 15 minutes to 1.5 hours, most preferably from 30 minutes to one hour.

A smoking article preferably is made using the prepared tobacco. If the smoking article is a cigarette, then ventilation holes preferably are introduced. If the cigarette contains a paper, then ventilation holes preferably introduced in the paper. If the cigarette contains a filter, then ventilation holes preferably are introduced in the filter. If the cigarette contains a paper and a filter, then ventilation holes preferably are introduced in the paper and/or the filter.

The present invention also relates to tobacco and smoking articles, such as cigarettes, prepared by any of the above-described methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table which shows data demonstrating that treating tobacco with either the transition metal salt Pd(OOCCH₃)₂ or Pd(NO₃)₂ results in the reduction in the levels of four PAHs in mainstream smoke. The term “Hold Time” refers to the length of time between the completion of spraying and the commencement of drying. The term “Solvent/Tobacco” refers to the weight of the solution added to the tobacco divided by the “as is” weight of the tobacco to which the solution is added. The term “OV” means oven drying for one hour or less at 60° C. The term “NE” means natural evaporation. The term “FA” means forced air having a relative humidity of 60%.

FIG. 2 is a graph illustrating the reduction in the level of BaP in the mainstream smoke of tobacco treated with a Pd(OOCCH₃)₂ solution as a function of the concentration of palladium in the treated tobacco.

FIG. 3 is a table which shows data demonstrating that mannitol and urea enhance the ability of a palladium salt to reduce the level of BaP in mainstream smoke.

FIG. 4A is an illustration of a cigarette having four ventilation holes in the cigarette paper, and FIG. 4B is an illustration of a cigarette having four ventilation holes in the cigarette filter.

FIG. 5 is a graph which shows data demonstrating that ventilation of either the cigarette paper or the cigarette filter enhances the ability of a palladium salt to reduce the level of BaP in mainstream smoke.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT

The following examples show that treatment of tobacco with a solution containing a palladium salt reduces the level of at least one PAH in the mainstream smoke of the treated tobacco. These examples also demonstrate that this effect is enhanced by the addition of mannitol and urea to the solution, and by ventilating the paper or filter of a cigarette containing treated tobacco. These examples are set forth by way of illustration only, and nothing therein shall be taken as a limitation upon the overall scope of the invention.

EXAMPLE 1

Tobacco was treated by spraying either a Pd(OOCCH₃)₂ solution or a Pd(NO₃)₂ solution onto “as is” tobacco. The tobacco was then dried. The concentration of metal in the resultant treated tobacco was 0.05% (w/w). The Pd(NO₃)₂ solution was prepared by dissolving the salt in water. The Pd(OOCCH₃)₂ solution was prepared by dissolving the salt in one part of acetone, followed by the addition of nine parts of water. Subsequently, the mainstream smoke of treated and untreated tobacco was analyzed for the presence of the following PAHs: phenanthrene, benzo(a)antracene (BaA), pyrene, and BaP. The percent reduction in the level of each of these PAHs was calculated using the following formula: 100(1-(level of the PAH in mainstream smoke of treated tobacco/level of the PAH in mainstream smoke of untreated tobacco)), wherein the PAH is phenanthrene, BaA, pyrene, or BaP. The results of these experiments are given in FIG. 1.

EXAMPLE 2

Tobacco was treated by spraying either a Pd(OOCCH₃)₂ solution or a Cu(OOCCH₃)₂•H₂O solution onto “as is” tobacco. The tobacco was then dried. The Pd(OOCCH₃)₂ solution was prepared as described in Example 1. The Cu(OOCCH₃)₂•H₂O solution was prepared by dissolving the salt in water. Subsequently, the mainstream smoke of treated and untreated tobacco was analyzed for the presence of BaP and other PAHs. The percent reduction in BaP level was calculated using the following formula: 100(1-(level of BaP in mainstream smoke of treated tobacco/level of BaP in mainstream smoke of untreated tobacco)). The results of these experiments are given in FIG. 2.

EXAMPLE 4

Tobacco was treated by spraying onto “as is” tobacco either a Pd(OOCCH₃)₂ solution or a Pd(OOCCH₃)₂ solution additionally containing mannitol and urea. The tobacco was then dried. The concentration of palladium in the resultant treated tobacco was 1.4% (w/w). The concentrations of mannitol and urea in the resultant treated tobacco containing these compounds was 10% and 5% (w/w), respectively. Subsequently, the mainstream smoke of treated and untreated tobacco was analyzed for the presence of BaP. The percent reduction in BaP level was calculated using the formula noted in Example 2. The results of these experiments are given in FIG. 3.

EXAMPLE 5

Tobacco was treated by spraying a Pd(OOCCH₃)₂ solution onto “as is” tobacco. The tobacco was then dried. The concentration of palladium in the resultant treated tobacco was 1.4% (w/w). Cigarettes were prepared using the treated tobacco and cigarette paper having a porosity of 33 CORESTA units. Three types of cigarettes were prepared: (1) unventilated cigarettes, (2) cigarettes ventilated in the cigarette paper, with each cigarette having four 0.5 mm-diameter holes approximately 6 mm apart (see FIG. 4A), and (3) cigarettes ventilated in the cigarette filter, with each filter having four 0.5 mm-diameter holes approximately 5 mm apart (see FIG. 4B). Subsequently, the mainstream smoke from each of the three types of cigarettes was analyzed for the presence of BaP. Percent reduction in BaP level was calculated using the formula noted in Example 2. The results of these experiments are given in FIG. 5.

Other features of the present invention are discussed below.

X-ray Absorption Fine Structure was used to study the catalytic effect of metal on the thermal decomposition of cellulose as a function of termperature. Microcrystalline cellulose powder containing 1% of metal was prepared using Pd(OOCCH₃)₂ as the metal source. The resulting samples were pyrolyzed isothermally in helium, for 10 minutes, at temperatures ranging from 150° C. to 600° C. High Resolution Transmission Electron Microscope (HRTEM) indicated the presence of a relatively uniform particle size distribution in the low nanometer range. The XAFS spectra of the solid residues obtained after the thermal treatment collected at the Pd K-edge. As expected, the unpyrolyzed cellulose Pd acetate samples demonstrate oxygen as nearest neighbors to metal, since the acetate ligand of the salt remains intact. Pure Pd(OOCCH₃)₂ undergoes thermal decomposition, in helium, between 200° C. and 300° C. However, the presence of Pd clearly changes the thermal behavior of cellulose over the temperature range of studies. The preliminary XAFS data analysis of the charred samples indicates that palladium got reduced to its metallic.

The effect of inorganic salts on the pyrolysis of biomass materials has been extensively studied. It is well known that the presence of inorganic salts can lower the primary pyrolysis temperature, increase the char yield by accelerating decomposition reactions, increase or decrease the rate of oxidation of the aromatic component of the char, and increase or decrease the heat released from the oxidation of char to CO₂. Although a wealth of information exists, relatively little is known about the effect of, and the fate of, palladium during the thermal decomposition of biomass materials. The ultimate goal of this project is to study the thermal behavior of cellulose in the presence of Pd. To meet this goal, a precise knowledge of the atomic environment of Pd is needed. X-ray Absorption Fine Structure (XAFS) is a powerful technique in characterization of catalyst both in its static and dynamic state (in-situ conditions). It can provide the interatomic distances and the coordination numbers between a particular atom and the surrounding atoms in the first, second and third coordination shells. A great advantage of the XAFS technique in relation to catalyst characterization is the ability to perform measurements under realistic conditions. Suitably designed in-situ reaction chambers will allow us to study a working catalyst at elevated temperature and pressure.

In an effort to explore the effect of Pd salts on biomass decomposition, the effet of Pd(OOCH₃)₂ on the decomposition of Avicel cellulose has been investigated. Cellulose samples loaded with 1% w/w Pd were pyrolzyed in helium at temperatures ranging from 150 to 600° C. The fate of Pd during pyrolysis was monitored using ex-situ XAFS and High Resolution Electron Microscopy (HRTEM). The major components of collected pyrolysis tars were characterized using gas chromatography—mass spectroscopy (GC-MS). This short manuscript focuses on the ex-situ XAFS measurements on Pd(OOCH₃)₂. The results demonstrate the usefulness of the technique in helping to understand the role of Pd during cellulose decomposition.

Microcyrstalline cellulose powder containing 1 w/w% palladium was prepared using palladium acetate as the source of metal. Cellulose and acetone solution of Pd(OOCCH3)2 were combined into a slurry, followed by solvent evaporation at room temperature. This procedure allowed maximum dispersion and minimal reduction of palladium. The char samples were obtained by pyrolysis of the cellulose/palladium acetate mixture at various temperatures in a quartz reactor according to the experimental design of McGrath, et al. For each temperature the initial mixture was pyrolyzed for 10 minutes under constant helium flow of 120 N cm3/min at atmospheric pressure.

A Philip's FEI Techni field emission high resolution transmission electron microscope (HRTEM) was used to estimate the morphology the Pd particles. The image of the unpyrolyzed starting material is shown in FIG. 1. The fast Fourier transform (FFT) indicated many reflections that could not be indexed as Pd metal, which were probably palladium acetate. Heating of the sample to 300° C. seemed to reduce Pd to its metallic state. The FFT of the image demonstrated d-spacing corresponding to Pd metal only (FIG. 2) with particle sized smaller than 5 nm. TEM images of the sample pyrolyzed at 600° C. indicated a typical particle size of 10 nm or greater. Selected area electron diffraction patterns revealed that these particles consisted exclusively of palladium metal.

A gas chromatograph (GC) coupled to a mass spectrometer (MS) was used to analyze the condensable volatiles (tar) to understand the mechanism of thermal decomposition of cellulose in the presence of the Pd salt. Changes in the chemical composition of the tar and in the weight loss as a function of temperature provide evidence that the presence of palladium modifies the thermal behavior of cellulose.

XAFS measurements were carried out at the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory at beam-line X11A. The x-ray beam was monochromatized by using a Si(111) double crystal monochromator. The storage ring was operated at 2.8 GeV with a maximum beam current of 300 mA. XAFS spectra were collected, at room temperature, at Pd K-edge, in both fluorescence and transmission mode. The energy of monochromatized beam was calibrated using a pre-edge peak of a 5 nm thick Pd foil.

XAFS data was reduced and analyzed using the data analysis package developed at the University of Washington. The pre edge and background were subtracted from the normalized data set. The resulting absorption spectra were averaged before further analysis.

The magnitude of Fourier transform of the oscillatory parts, extracted from the XAFS absorption spectra of samples pyrolyzed at various temperatures and the two reference materials (unpyrolized sample and Pd foil), is shown in FIG. 3. The simple theoretical formulation given by Sayers et al. showed that the Fourier transform of the XAFS signal gives information of the radial distribution function around the X-ray absorbing atom. Further analysis reveals the information on near neighbor distances, the coordination number of the central atom and the information about the surrounding atoms. From the qualitative analysis of the transforms of the pyrolized samples, as shown in FIG. 3, a significant change in the Pd environment took place between 150° C. and 200° C. A smaller change in the local structure was observed for samples pyrolyzed between 200° C. and 400° C. After pyrolyzing at 600° C., the Pd present in the sample is almost completely reached to the metallic state. FIG. 4 shows the R space Fourier transform of the Pd K-edge XAFS data of the 250° C. sample. The transforms were taken at k=1.5 Å⁻¹ and k=11.5 Å⁻¹ with k² weighting. The R space fits were performed over the first shell between R=1.2 A and R=3.0 A with no phase correction. Metallic Pd-Pd and Pd-O contributions of Pd K-edge Fourier transofmrs are fitted by two shell fits with five variable. The average coordination number n (Pd-Pd), n (Pd-O), average interatomic distances d (Pd-Pd), d (Pd-O), thermal disorder σ² (Pd-O) were varied. Other parameters were set fixed to the values of standards. Cubic close-packed palladium metal (space group Fm-3m) and PdO (space group P4/nmm) were used to fit the first shell XAFS data. The structural information derived from XAFS analysis on Pd-O coordination with Pd as the central atom is shown in Table 1. Here n is the average percentage of first shell oxygen atom around Pd atom, in samples pyrolyzed at various temperatures. As the temperature changes from 150° C. to 200° C., oxygen concentration decreases from 67% to 35%. At 600° C. only 2% of the atoms around Pd are oxygen. After fitting the data with standards, the Pd-O distances in the samples, were found to range between 1.973-2.014 Å, which is consistent with the published results.

XAFS data will be taken at lower temperature (77K) to reduce thermal disorder in XAFS measurements.

In-situ XAFS measurements will be used to elucidate the catalytic performance of the metal. The structures of catalysts are greatly dependent on the reaction condition due to direct chemical interaction with the reactants. Thus, information on the nature of active catalytic sites under reaction condition is essential for designing better catalysts. XAFS is an ideal tool for structural analysis of active sites under the reaction conditions, because X-rays are not generally interfered with by the reaction gases present in the catalytic system.

Temperature (° C.) n Δn d_(Pd-O)(Å) Δd(Å) Unpyrolyzed 0.890 +/−0.0573 1.986 +/−0.0053 150 0.670 +/−0.0773 1.960 +/−0.0160 200 0.351 +/−0.0525 1.935 +/−0.0260 250 0.310 +/−0.0426 2.015 +/−0.0310 300 0.278 +/−0.058  2.001 +/−0.0320 350 0.288 +/−0.0431 1.979 +/−0.0380 400 0.246 +/−0.0394 1.976 +/−0.0400 600 0.019 +/−0.0388 2.021 +/−0.0600

Table 1: Structural information derived from XAFS analysis on Pd-O coordination with Pd as the central atom. Here n is the average percentage of first shell oxygen atom around Pd atom, in samples pyrolyzed at various temperatures. After heating at 600° C. palladium is predominantly reduced to its metallic state. 

1. A method for preparing tobacco comprising adding to tobacco a solution containing a palladium salt.
 2. The method of claim 1, wherein the anion of the palladium salt is nitrate, chloride, or the anion of an organic acid.
 3. The method of claim 2, wherein the anion of the organic acid is acetate, citrate, gluconate, or salicylate.
 4. The method of claim 1, wherein the solvent in the solution is water, acetone, ethanol, a mixture of water and acetone, or a mixture of water and ethanol.
 5. The method of claim 4, wherein the concentration of acetone in the mixture of water and acetone is from 5% to 15% (v/v), and wherein the concentration of ethanol in the mixture of water and ethanol is from 5% to 15% (v/v).
 6. The method of claim 1, wherein the concentration of palladium in the prepared tobacco is from 0.01% to 0.2% (w/w).
 7. The method of claim 1, wherein the concentration of palladium in the prepared tobacco is from 0.02% to 0.07% (w/w).
 8. The method of claim 1, wherein the solution further contains mannitol.
 9. The method of claim 8, wherein the concentration of mannitol in the prepared tobacco is from 0.5% to 20% (w/w).
 10. The method of claim 8, wherein the concentration of mannitol in the prepared tobacco is from 8% to 12% (w/w).
 11. The method of claim 1, wherein the solution further contains urea.
 12. The method of claim 11, wherein the concentration of urea in the prepared tobacco is from 0.5% to 10% (w/w).
 13. The method of claim 11, wherein the concentration of urea in the prepared tobacco is from 2% to 5% (w/w).
 14. The method of claim 1, wherein the solution contains mannitol and urea.
 15. The method of claim 1, wherein the weight of the solution added to the tobacco divided by the weight of the tobacco to which the solution is added is from 0.2 to 1.0.
 16. The method of claim 1, wherein the weight of the solution added to the tobacco divided by the weight of the tobacco to which the solution is added is from 0.4 to 0.9.
 17. The method of claim 1, wherein the weight of the solution added to the tobacco divided by the weight of the tobacco to which the solution is added is from 0.6 to 0.9.
 18. The method of claim 1, wherein the solution is added to the tobacco by spraying.
 19. The method of claim 1 further comprising drying the tobacco after adding the solution to the tobacco.
 20. The method of claim 19, wherein the drying is done by forced air or natural evaporation.
 21. The method of claim 19, wherein the drying is done by forced air, and wherein the forced air has a relative humidity of from 50% to 70%.
 22. The method of claim 19, wherein a finite period of time is allowed to elapse between adding the solution to the tobacco and drying the tobacco.
 23. The method of claim 22, wherein the finite period of time is from 5 minutes to 24 hours.
 24. The method of claim 22, wherein the finite period of time is from 15 minutes to 3 hours.
 25. The method of claim 22, wherein the finite period of time is from 15 minutes to 1.5 hours.
 26. The method of claim 22, wherein the finite period of time is from 30 minutes to one hour.
 27. The method of claim 1 further comprising making a smoking article containing the prepared tobacco.
 28. The method of claim 27, wherein the smoking article is a cigarette.
 29. The method of claim 28, wherein the cigarette contains a cigarette paper.
 30. The method of claim 29 further comprising introducing ventilation holes in the cigarette paper.
 31. The method of claim 29, wherein the cigarette contains a filter, and the method further comprises introducing ventilation holes in the cigarette paper or the filter.
 32. Tobacco prepared by the method of claim
 1. 33. A smoking article containing the tobacco of claim
 32. 34. The smoking article of claim 33, wherein the smoking article is a cigarette.
 35. The smoking article of claim 33, wherein the cigarette contains a cigarette paper.
 36. The smoking article of claim 35, wherein the cigarette paper contains ventilation holes.
 37. The smoking article of claim 35, wherein the cigarette contains a filter, and wherein the cigarette paper or the filter contains ventilation holes. 